The present invention relates to a steam turbine plant and, more particularly, to a control method and apparatus for operating a feedwater heater of a steam turbine plant which enables an increase in a useful service life of the feedwater heater of the steam turbine plant. Steam turbine power plants are widely used for medium loads which require frequent starts and shutdowns such as a daily start and shutdown operation. With this type of operation of power plants, a wall member of a water chamber in a feedwater heater, especially in a high-pressure feedwater heater, is subjected to an abrupt increase or decrease in temperature caused by a sharp of steep and large load change required during starting or shutdown operations of the steam turbine plant. Consequently, a considerable thermal stress occurs at least partially in the wall member of the water chamber in the feedwater heater, and a repetition of the subjecting of the wall member to the large thermal stresses substantially reduces the life span of the metal of the wall forming the water chamber in the feedwater heater, thereby resulting in a premature damaging of the feedwater heater.
If the wall of the water chamber is made thicker in proportion to the higher pressure necessary for applying a super-critical pressure in a steam turbine power plant, larger thermal stresses are caused during a starting or stopping operation of the steam turbine power plant, with the thermal stresses being extreme and resulting in a damaging of the high pressure feedwater heater.
In, for example, Japanese Patent Laid Open Application No. 1905007/1984, a steam turbine power plant is proposed having a steam generator and a warming or heating pipe means for connecting a high pressure feedwater heater and a steam generator for warming the high pressure feedwater heater prior to a starting and stopping or shutdown of the steam turbine plant, so as to reduce the thermal stress on the high pressure feedwater heater thereby increasing the service life of the feedwater heater.
A disadvantage of the above proposed construction resides in the fact that it is necessary to provide a steam generator and a warming or heating pipe means for generating the high temperature steam and for introducing the steam in order to heat or warm the high pressure feedwater heater whenever the plant is started and stopped. Consequently, the construction of the above proposed steam turbine plant is considerably large and extremely complicated.
The aim underlying the present invention essentially resides in providing a steam turbine power plant with a feedwater heater, which power plant includes means for enabling a temperature control of the feedwater heater without an additional steam generator and/or warming pipe means and which seeks to increase the service life of the feedwater heater.
In accordance with advantageous features of the present invention, thermal stress in the feedwater heater is reduced at an adequate range during operation of the starting and stopping or shutdown of the steam turbine plant in order to prevent damage or consumption of the feedwater heater thereby increasing the service life thereof.
Additionally, in accordance with the present invention, the reliability of the feedwater heater of the steam turbine plant may be significantly increased.
In accordance with the present invention, a steam turbine plant is provided which includes a boiler, a steam turbine, having at least one steam extracting pipe means, and a feedwater heater means connected with the steam extracting pipe means and disposed in the feedwater system of the steam turbine plant. Means are provided for regulating an extracting steam flow rate, with the regulating or control means being adapted to control the steam flowing into the feedwater heater at a suitable steam condition when the steam turbine plant is operating for a starting and shutdown operation.
By virtue of the features of the present invention, it is possible to increase the service or consumption life of the feedwater heater, and also improve the reliability of the steam turbine plant.